Conventionally, a metal, e.g., copper (Cu), is used for a wiring of a three-dimensional (3D) stacked memory. In an ultra-fine wiring structure formed of a metal wiring material such as Cu, conduction electrons are intensely affected by an inelastic scattering in an interface due to a fine line effect, resulting in a problem that a wiring has a high resistance.
Meanwhile, graphene has a very long mean free path or a high mobility. When the grapheme is applied to a fine wiring structure, a low resistance wiring exceeding Cu can be implemented. Thus, in a next-generation 3D stacked memory required to realize a finer stacked structure or wiring structure, the use of graphene, instead of Cu, in a wiring film is under consideration.
A chemical vapor deposition (CVD) (e.g., thermal CVD or plasma CVD) method which is a typical graphene producing method, includes covering a surface of a substrate with a catalytic metal layer, activating the catalytic metal layer, dissolving carbon atoms decomposed from a raw material gas into the activated catalytic metal layer, and recrystallizing the carbon atoms. That is to say, the CVD method is capable of easily producing graphene on a substrate having a relatively large area, thus being easily adapted to an existing semiconductor device formation process.
In the thermal CVD of the CVD method, it is necessary to heat a substrate to about 1,000 degrees C. so as to thermally decompose a raw material gas, thus causing a concern that another wiring film or insulating film in the 3D stacked memory is modified. As such, as it now stands, the plasma CVD which decomposes the raw material gas by plasma and heats the substrate at a relatively low temperature, e.g., 600 degrees C. or lower, is mainly used. In the plasma CVD method, for example, a hydrocarbon-based gas is used as the raw material gas. Plasma is generated from the hydrocarbon-based gas, and carbon radicals in the generated plasma are dissolved into a catalytic metal layer.
However, when graphene is used for a wiring film, it is important to increase the size of the domain (mass) of graphene as a crystalline body in order to enhance conductivity of graphene. In particular, it was confirmed that, in a low temperature growth mode in which graphene is produced at 600 degrees C. or lower, domains of graphene grow from facets as a specific miller index plane in a crystal of a catalytic metal.
In each facet, cores (nuclei) of graphene are generated at an initial production stage of grapheme. Since the cores are starting points at which grapheme grows, a decrease in the number of facets decreases the number of the growth starting points of grapheme. As a result, it is possible to secure an expansion space of each domain, thus increasing a size of each domain.
However, the decrease in the number of facets and the number of the growth starting points of graphene requires a period of time during which the entire grapheme is grown. As such, an amount of heat applied to the graphene while being produced is increased so that a surface roughness of grapheme is increased, thus decreasing the expansion space of each domain. This fails to increase the size of each domain.
Meanwhile, if a high reactivity hydrocarbon-based gas is used to accelerate the growth of the entire graphene, cores of graphene are generated even in areas other than each facet of a catalytic metal layer at the initial production stage of graphene, increasing the number of cores. As a result, the number of generated domains is increased, which makes it impossible to secure the expansion space of each domain, decreasing the size of each domain.